Heretofore, as a cable termination of this type, one constructed as shown in FIG. 8 has been known. Referring to the figure, the cable termination in the prior art includes a cable terminal 20, and a porcelain jacket 21 which envelops the cable terminal 20 and which is filled with an insulating compound 22 such as insulating oil or insulating gas.
Here, an annular bottom metal fitting 23 is mounted on the bottom part of the porcelain jacket 21, and an epoxy seat 24 is disposed concentrically with the porcelain jacket 21 at the lower part of the interior of the porcelain jacket 21. Besides, an upper metal fitting 24 and an upper covering 25 are respectively disposed at the top part of the porcelain jacket 21, and a conductor draw-out bar 26 is disposed concentrically with the porcelain jacket 21 at the upper central part of interior of the porcelain jacket 21. Incidentally, the lower end part of the conductor draw-out bar 26 is located within the porcelain jacket 21, and the distal end part thereof is gastightly led out upwards from the central parts of the upper metal fitting 24 and the upper covering 25.
Besides, a stress cone 28 is mounted on the outer periphery of a cable insulator 27 constituting the cable terminal 20, and a cable conductor 29 is attached to the distal end part of a cable conductor (not shown). By the way, in the figure, numeral 30 designates a pressing device which presses the stress cone 28; each of signs 31a and 31b, a sealing portion; numeral 32, a clamp metal fitting; numeral 33, a supporting insulator; and numeral 34, a supporting stand.
In the cable termination of such a construction, the conductor terminal 29 is connected to that lower end part of the conductor draw-out bar 26 which is located within the porcelain jacket 21, and the outer surface of the stress cone 28 is held in pressed contact with the inwall surface of the epoxy seat 24.
In the cable termination of such a construction, however, the connection node between the conductor terminal 29 and the conductor draw-out bar 26 exists in the interior of the porcelain jacket 21, and there have been drawbacks as stated below.
In the first place, there has been the drawback that, since the epoxy seat for receiving the stress cone exists inside the porcelain jacket, the diameter of the porcelain jacket enlarges accordingly, resulting in the heavy weight of the porcelain jacket.
Secondly, there has been the drawback that, when the outside diameter of the porcelain jacket enlarges, the projection cross-sectional area thereof becomes large, resulting in the degradation of the characteristic of the pollution withstand voltage of the porcelain jacket, so an elongated porcelain jacket must be used to keep a predetermined characteristic of pollution withstand voltage.
Thirdly, the porcelain jacket is filled with the insulating compound, so that when the porcelain jacket has broken down, the insulating compound might flow out of this porcelain jacket to cause a secondary disaster.
Fourthly, there has been the drawback that, since the components such as epoxy seat, upper covering and clamp metal fitting are required, a large number of components and a complicated structure are involved.
In view of these drawbacks, there have been developed (I) a cable termination wherein a polymer bushing is used instead of the porcelain jacket 21 shown in FIG. 8, and wherein the polymer bushing is filled with an insulating oil or insulating gas, and (II) a cable termination wherein, as shown in FIG. 9, a cable terminal 40 is mounted in a receiving port 46 of a polymer bushing 41 which includes a conductor draw-out bar 42 that is centrally disposed, a hard insulation sleeve 43 that is disposed on the outer periphery of the conductor draw-out bar 42, and a polymer clad body 44 that is provided integrally with the outer periphery of the insulation sleeve 43.
In the cable termination (I), however, the bushing is formed of a polymer being a high-molecular material, so that a water content might penetrate therethrough from outside to mix into the insulating oil or insulating gas contained in the polymer bushing, and to deteriorate the performance of the insulating oil or insulating gas. Besides, in the cable termination (II), any epoxy seat for receiving a stress cone does not exist inside the polymer bushing 41, and this polymer bushing 41 need not be filled with any insulating compound, so that the structure of the polymer bushing 41 can be simplified, and the weight thereof can be lightened. As further merits, the individual portions can be put into the form of units and transported to a site, and the on-site execution time thereof can be shortened. Since, however, the connection node between the cable conductor 45 of the cable terminal 40 and the conductor draw-out bar 42 exists in the interior of the polymer bushing 41, there has been the drawback that, as in <the porcelain jacket stated before, the outside diameter of the polymer bushing 41 enlarges to make the weight thereof heavy. Besides, when the diameter of the polymer bushing 41 enlarges, the projected cross-sectional area thereof becomes large. This results in the drawback that an elongated polymer bushing must be used for attaining enhancement in the characteristic of pollution withstand voltage.
Therefore, the Applicant has developed a polymer bushing 56 and filed an application before (Japanese Patent Application No. 2002-105432). As shown in FIG. 10, the polymer bushing 56 includes a hard insulation sleeve 53 which has a conductor draw-out bar 51 at the center thereof, and a receiving port 52 for a cable terminal at the lower end part thereof, and a polymer clad body 55 which is provided integrally with the outer periphery of the insulation sleeve 53 and which is formed with a large number of shades 54 at its own periphery in a manner to be spaced from one another in its longitudinal direction. Herein, the receiving port 52 is located at a position lower than the polymer clad body 55.
The polymer bushing 56 of such a construction has the following merits: In the first place, since the polymer clad body is provided integrally with the outer periphery of the hard insulation sleeve, the polymer bushing is lighter in weight and is less liable to damage than the prior-art porcelain jacket, and the handling of the polymer bushing can be more facilitated to sharply enhance a working property. Secondly, since any insulating oil or insulating gas is not required, the harmony of an environment can be attained. Thirdly, in a case where the polymer clad body is formed of a silicone polymer, the characteristic of pollution withstand voltage can be enhanced owing to the water repellency of the silicone polymer. Fourthly, since the receiving port of the insulation sleeve is located at the position lower than the polymer clad body, the polymer bushing can be made finer than the prior-art polymer bushing, and in consequence of the finer polymer bushing, the projection cross-sectional area of the polymer bushing becomes small, with the result that even a short polymer bushing can keep a predetermined characteristic of pollution withstand voltage. Fifthly, since the connection between the conductor draw-out bar and the cable conductor is made at a position lower than the polymer clad body, the step-stripped processing section of the cable terminal can be shortened. Sixthly, since an embedment metal fitting is embedded in the polymer bushing and is made integral with a bottom metal fitting, the polymer bushing can be mechanically reinforced, and it can be easily and stably mounted on a mounting stand or the like through the bottom metal fitting.
In the polymer bushing 55 of such a construction, 6 however, drawbacks as stated below have been involved.
In the first place, the thickness of the polymer bushing 55 is determined by the electric field of the outer surface of the polymer bushing 55 rather than by the allowable electric field of the insulation sleeve 53 of epoxy or the like, so that as the service voltage of a cable termination becomes higher, the insulator of the insulation sleeve 53 thickens more to consequently make the polymer bushing 55 heavier.
Secondly, as the service voltage of the cable termination becomes higher, the polymer bushing 55 is lengthened more, and the handling of the polymer bushing 55 in an installation work becomes more difficult due to the larger length conjointly with the heavier weight.
[Patent Document 1]
Japanese Utility Model No. 3,031,834 (Sector Nos. “0002”-“0004”, FIG. 2)